Tewksbury is a town in Middlesex County, Massachusetts, United States. The population was 28,961 at the 2010 census.

According to the United States Census Bureau, the town has a total area of 21.1 square miles (55 km2), of which 20.7 square miles (54 km2) is land and 0.3 square miles (0.78 km2), or 1.61%, is water. The Merrimack River forms part of the northern boundary of Tewksbury, and the Shawsheen River runs through the southern end of town as well. Tewksbury is in Greater Lowell. The town is located about 19 miles (31 km) north-northwest of Boston along I-93 and I-495 (Boston's outer beltway). Tewksbury is bordered by the city of Lowell to the northwest, Dracut to the north (unreachable across the Merrimack), Andover to the northeast, Wilmington to the southeast, and Billerica to the southwest. Tewksbury also meets the town of Chelmsford at a point in the middle of the Concord River along with Lowell and Billerica.

As of the census of 2000, there were 28,851 people, 9,964 households, and 7,692 families residing in the town. The population density was 1,392.3 people per square mile (537.6/km≤). There were 10,158 housing units at an average density of 490.2 per square mile (189.3/km≤).